Cell junctions are specialised junctions between cells. They occur within animal tissues (e.g. animal tissues, nerve tissues, etc.). They are formed by multiprotein complexes that provide contact between adjacent cells or between a cell and the extracellular matrix. There are three major types of cell junctions: (1) tight junctions, (2) gap junctions, and (3) anchoring junctions (e.g. desmosomes).

A tight junction is a type of cell junction wherein the membranes of two neighboring cells join to form a barrier to larger molecules and water that pass between the cells. As a barrier, tight junctions help maintain the polarity of cells and the osmotic balance. Tight junctions also play a role in maintaining blood-brain barrier. Apart from serving as a barrier between cells, tight junctions also function as a means to hold cells together.

Epithelial cells with tight junctions may be described as tight or leaky depending on the presence and the ability of tight junctions to prevent water and solute movements.